1. Field of the Invention
The present invention relates to a toner, a developer, and an image forming apparatus.
2. Description of the Related Art
Conventionally, in an electrophotographic apparatus, an electrostatic recording apparatus, etc., an electrical or magnetic latent image is visualized with a toner. For example, in electrophotography, an electrostatic image (latent image) is formed on a photoconductor, then the latent image is developed using a toner, and a toner image is formed. The toner image is usually transferred on a recording medium such as paper, and then fixed by a method such as heating.
A resin binder accounts for over 70% of the components of such a toner, most of which uses petroleum resources as a raw material. Problems such as exhaustion of petroleum resources in the future and global warming caused by carbon dioxide discharged into the atmosphere due to mass consumption of petroleum resources are concerned. Thus, if a toner binder is made of an environmental cycling polymer that a plant which grows by taking in carbon dioxide in an atmosphere, carbon dioxide generated thereby merely circulates in the environment. Thus, suppression of global warming and depletion of petroleum resources may be simultaneously solved. Accordingly, a polymer made of botanical resources (biomass) is gaining attention.
As an attempt to use such a plant-derived resin as a toner binder, for example, in Japanese Patent (JP-B) No. 2909873, use of a polylactic acid as a binder resin is proposed. This polylactic acid is readily available for general use as a polymer made of botanical resources, and it is synthesized by dehydration condensation of a lactic acid monomer or by ring-opening polymerization of a cyclic lactide of a lactic acid (see JP-B No. 3347406 and Japanese Patent Application Laid-Open (JP-A) No. 59-96123). However, when the polylactic acid is used for a toner as it is, it is difficult to obtain all the physical properties required for a toner only by the polylactic acid because it has high ester group concentration compared to a polyester resin and a molecular chain which is through an ester bond is a carbon atom only (N=1).
To solve this, it is considered to ensure physical properties and thermal properties required for a toner by (1) mixing the polylactic acid and a second resin other than the polylactic acid, or by (2) copolymerizing the polylactic acid. To improve the thermal properties, for example, in JP-B No. 3785011, inclusion of a terpene phenol copolymer in a polylactic acid resin as a low-molecular-weight component is proposed. However, this proposal does not satisfy both low-temperature fixing property and hot offset property, and has not yet been put into practical use. Also, the polylactic acid resin has extremely poor compatibility or dispersibility with a polyester resin and a styrene-acrylic copolymer commonly used for a toner. In the case of combining it with other resins, controlling a composition of an outermost surface which assumes important features of the toner such as storage stability, charging property and fluidity becomes extremely difficult.
Meanwhile, as an example of an attempt to solve the problem by copolymerization, a block copolymerization resin with a polyester resin other than a polylactic acid skeleton of which a D/L ratio of polylactic acids are defined is proposed (JP-A No. 2008-262179). Since about half or more of energy consumption in an electrophotographic image forming apparatus is consumed for heating a toner in a thermal fixing system, market demand not only for a fixing apparatus with low-power consumption but also for a toner which enables low-temperature fixing has further increased in recent years. To this demand, satisfactory properties cannot be obtained with the toner of JP-A No. 2008-262179, and even with a polymer made of botanical resources, improvement for further low-temperature fixing property has been desired.
Also, among toner properties, low-temperature fixing property and heat-resistant storage stability have a trade-off relationship, and there is a problem that decreasing thermal properties for achieving low-temperature fixing property involves degradation of heat-resistant storage stability. For example, in JP-A No. 2004-310018, a high-molecular-weight polyester resin obtained by elongation of a prepolymer is used in combination with a low-molecular-weight polyester resin as a binder resin of a toner. It is considered that the low-molecular-weight polyester resin contributes to low-temperature fixing property and that the high-molecular-weight polyester resin contributes to hot-offset resistance and heat-resistant storage stability, but in reality, the high-molecular-weight polyester inhibits fixing of the toner to paper, and sufficient low-temperature fixing property cannot be obtained. Thus, it is difficult to obtain both low-temperature fixing property and heat-resistant storage stability of a toner simply by combining a low-molecular-weight resin and a high-molecular-weight resin as a binder resin of a toner.
Also, decreasing thermal properties for achieving low-temperature fixing property not only invites degradation of heat-resistant storage stability but also reduction of resin hardness. Reduction of resin hardness affects image quality since mechanical loads such as agitation and compression applies on a toner not only during long-term storage in a standing state but also as a mechanical load inside a printer during continuous printing.
Accordingly, a toner using a binder resin with a polylactic acid skeleton which has superior low-temperature fixing property and heat-resistant storage stability and provides less reduction of toner performance during continuous printing and related technologies thereof has not yet been obtained, and further improvement and development has been currently desired.